Selecting lens for IR thermometer

High school optics is better than good enough. You must have done some simple ray tracing for thin lenses back when, no?

The main problem is going to be what to make the lens out of. Germanium and silicon are the usual choices. You can also use a very thin Fresnel lens made of HDPE, as used in front porch lights. If you're doing this on a hobby budget, go down to the hardware store and get a $12 front porch light sensor and rip out the Fresnel lens. Those ones are segmented, so you have to pay attention where you cut it.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

The $20 Fresnel lensed IR thermometer at Harbor Frieght can be micro adjusted (aimed) pretty easily, and at 5cm, you could easily point it only at the device in question.

Place a piece of room temp (ambient) matte black painted aluminum in front of the target with like a 1cm hole in it at the target location. Make a set of successively smaller holed plates like that one.

With that, you can see just how well your device is already pointing.

With successively smaller holes in the ambient plate, that is.

Also, your device under test has to be hotter than ambient to be a proper pointing determination target, etc.

Why bother? Perfect, ROUND Fresnel lens already in place, not to mention the little sensor and circuit behind it. Jeez. $20

The street light illuminator/dimmer jobs are wide scan format Fresnel that will see side to side well, but not up and down.

This little job actually has correct optical characteristics.

He said it was already "filtered" so no need to choose Germanium or Pyrex or other IR specific lens media.

This is one mouse trap that there are already probably "best of class" versions made. So much so that they are practically ubiquitous like wristwatches.

Because they're completely opaque in the thermal IR. Try using your Harbor Freight gizmo to look at a wood stove through a piece of Plexiglass. You'll measure the temperature of the plexi. Same with window glass.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Thanks Phil and TheGlimmerMan; thank you very much for your suggestions and for putting me on the right track. I suppose this is simpler than I thought - no need to worry about Gaussian beam theory, field of view, or the numerical aperture of the lens. These were the questions in my head when I made the initial post.

One other question: If I am to use a C-mount lens holder for the lens, would I have to worry about the color that the lens holder is painted if the sensor and lens assembly are taken outdoors? I've noticed that most of these C-mount lens holders are painted black, but this might heat up if the thermometer and lens assembly is placed outside in the sun. Would it be beneficial to coat the outside of the lens holder with white paint? This would be the side of the lens holder facing the surface to be measured. (Paint containing barium sulfate or a similar chemical might work.) The lens would be inserted into the C- mount holder and the holder then screwed on the end of the tube. The IR sensor would be situated at the end of the tube, aligned with the optical axis of the lens. What color should be the inside of the tube?

So there are only two parts to this assembly: a lens holder and a tube with an appropriately-machined end that allows the lens holder to be screwed on the end of the tube. The IR thermometer is situated at the opposite end of the tube, mounted on a PCB. The tube is fastened to the PCB by a flange on the end of the pipe. The flange has holes, and it is secured to the PCB with mounting screws. An O-ring is placed under the flange to ensure that stray light does not enter the tube.

How about using a reflecting parabola or a segment of one?

tm

Good thing neither are used as sight 'windows' on a wood stove.

Yes, I made instruments that were meant to look through "windows".

I made them that looked at molten glass gobs as well... through a glass window. Usually not though. :-)

I made a 4" diameter tube that had a Gold mirror and a rifle stock and a rifle scope on it, and the power industry used them before imagers to look at transformers on the poles, and ground and to look at insulator strings on towers for excessive heat (of course). Saved a lot of up close transformer inspections.

The "window" on the front of that tube had to be there to keep out dust.

That window was "IR transparent" for the range we were looking at.

It was mere plastic film held on by the tube cap's interference fit and stretch. But it had a narrow spectral bandpass capacity.

The one(s) we sold to NASA with the 1000' focus had a Pyrex glass lens, IIRC. Looking for a burn through in the launch gas exhaust ports on the big pads. 10 foot spot at 1000 feet. It is a one inch diameter lens, and a 2mm diameter transducer window with a 2x2 mm bolometer 2 mm below that window in a can.

We had solid Ge lens, but also had bolometers which had various "windows" right on the transducer, which is, of course, the cheapest route. Best of class. He stated his already has a filter on it. It is probably at the transducer level, and if the range he gave is wide, further filtration would also mean further attenuation of the signal, so hopefully he can compensate for that.

Archie, I'm proud of you! Not a nasty aspersion or swearword in that whole post!

Would you like this to be another addenda to your recent assessment or a good start on your next one?

Desired spot size 3mm D:S ratio 6 Aren't there gonna be significant depth of field issues up that close? Assuming there's enough free area around the target to get that close.

The colour won't make any difference--all paint has high emissivity in the thermal IR. Even shiny metal won't do much--you aren't cooling your detector, so it's going to see a room-temperature ambient field regardless.

The key is to use a lens that's transparent in the thermal IR, which doesn't happen by accident!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Fuck off, stalk boy!

Keep posting, bitch. Maybe I will print them out and use them as templates for the tattoos I emblazon across your pathetic forehead.

It is

Thanks for your response; yes - you are right about the filter being at the transducer level. Since it has been added by Melexis on the MLX90614, and I did not select it myself, adding another lens will indeed attenuate the light.

r

s.

Thanks, Phil; and thanks also to all who responded. I've been looking at IR lenses from Newport, and I think that these might be the best way to go

catalog.aspx). For budget applications, perhaps the AMPX010 or AMPX013 would be most appropriate, but the other lenses sold by this company may be good as well.

Sensitivity will be better with the 25 mm diameter, 25 mm FL one.

Infrared sensitivity is all about how much solid angle the FOV subtends at the detector. It's just like a voltage divider: you get (image projected solid angle)/pi less sensitivity you'd get with uniform (Lambertian) illumination.

(Projected solid angle is cos theta integrated over the FOV--for a lens of a given NA, it's conveniently

Omega prime = pi NA**2.

For an f/1 lens (NA=0.5) you lose a factor of 4, whereas for f/1.5 (NA =

0.33) it's a factor of 9.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

=3D

Phil, thank you for posting this. I think that I follow what you are saying, and I was able to calculate the loss factors for a number of different lenses. (I like the voltage divider analogy.) If I follow this correctly, would a high numerical aperture lens be most appropriate for this application? Thus, the higher the numerical aperture of the lens, the more sensitive that it would be, and the loss factor is consequently lower? So by increasing the diameter of the lens and keeping the focal length constant, the sensitivity increases? Does the magnification of the lens (expressed as an effective f-number) play a role in any of this as well?

I suspect that IR sensors have fudge factors that account for the mix of object radiation and internal (ambient temp) radiation that the sensor sees. So if you change the lens, that will change too. I think it might amount to introducing an error into the emissivity correction.

John

Thanks John; I will have to assess this effect carefully during calibration of the IR thermometer.

Why not just sweep it across a point source of heat and see the response curve?

John